What final appearance do you want? Clear or slightly amber? High gloss or satin sheen?

How much ventilation is available where the topcoat will be applied? Will the topcoat regularly come in contact with food?

Some common types of topccoats:

Oils: Easy to apply wiping or brushing, these finishes deepen and enhance the wood's natural color but don't provide a lot of protection. Damaged spots can be repaired easily with simple reapplication. Best used on low-wear surfaces. Oil-based polyurethanes/varnishes: These finishes can be applied by wiping, brushing or spraying and offer a durable, protective surface. Generally have slight amber cast. Good ventilation, solvent clean-up required. Repairs require sanding and recoating. Good for moderate- to high-wear surfaces. Water-based polyurethanes/varnishes: Also can be applied by wiping, brushing or spraying for a durable, protective surface. Generally dry clear, without amber cast. Low odor and water clean-up. Repairs require sanding and recoating. Good for moderate- to high-wear surfaces. Lacquer: Usually applied by spraying, lacquer quickly dries to a hard finish that can be polished to a high gloss and offers good protection against water damage and stains. Damage can be repaired easily. Good ventilation, solvent clean-up required. A good choice for surfaces that will see moderate wear.Shellac: A natural product that's non-toxic when dry, it can be applied by wiping, brushing or spraying and provides decent surface protection. Damaged spots can be repaired easily with simple reapplication. Often used as a seal coat. Best for low- to moderate-wear surfaces.